For this fiscal year, our primary effort has been to investigate the role of oxidative guanine and uracil in telomeres. We have characterized the impact of oxidized guanine and uracil and the role of their repair enzymes, 8oxoguanine DNA glycosylase 1 (OGG1) and uracil DNA glycosylase (UNG) in telomeres, which are summarized as follows: (1) Impact of oxidized guanine lesions on telomeres: We found that oxidized guanine lesions accumulated in telomeres in Ogg1 deficient yeast and primary embryonic fibroblasts (MEFs). These base lesions led to changes in telomere length in both yeast and mouse cells. Thus, OGG1 is critical in excising telomeric oxidized guanines and plays an important role in telomere length maintenance. (2) Impact of uracil base modification on telomeres: We found that Ung deficiency led to telomere attrition in mouse tissues and cells. These data suggest that uracil attenuates telomere integrity and UNG is a critical DNA glycosylase that excises telomeric uracil.[unreadable] [unreadable] The future work will test the hypothesis that telomeric DNA is vulnerable to oxidative damage; that telomere structure and telomere-associated proteins hinder BER at telomeres; and that oxidized base lesions in telomeres influence telomere length and/or function in aging. Three approaches will be used to explore this hypothesis: (1) We will determine the effect of oxidized base lesions on telomere integrity; (2) we will investigate mechanisms by which oxidized base lesions lead to telomere attrition; and (3) we will use mouse aging models to investigate whether oxidized base lesions contribute to aging-related symptoms. The overall effort will facilitate our understanding on how oxidative base lesions may impact telomere integrity and aging.